1. Field of the Invention
This invention relates to thermally conductive caulking compounds and a process for making said compounds.
2. Description of the Prior Art
A fuel cell power section comprises a plurality of individual cells electrically connected in series. The cells convert reactants such as hydrogen into DC electrical power in a manner well known in the art. Separator plates are disposed between individual cells and may be provided with flow passages to bring the reactants into the cells. The electrochemical reaction produces, as a by-product, heat which must be removed in a controlled manner to maintain the cells at the desired operating temperature. For efficient operation it is desirable to maintain the cells at a uniform temperature and at a maximum temperature level consistent with material compatibility characteristics.
A well known method for removing waste heat from a fuel cell power section is to use heat exchange surfaces parallel to the plane of the cells. These heat exchange surfaces often take the form of passageways through the separator plates; the passageways carry a coolant fluid. Intimate contact between the coolant fluid and the separator plates provides high heat transfer capabilities between the cells and the cooling medium thereby minimizing the temperature gradient therebetween. In commonly owned U.S. Pat. No. 3,880,670 to Byron H. Shinn the coolant is carried directly within these passageways, and contacts the surfaces of the passageways. Commonly owned U.S. Pat. No. 3,990,913 to Jerome G. Tuschner shows the coolant fluid being carried by tubes disposed within these passageways. This latter technique solves sealing and materials compatibility problems as between the coolant fluid and the fuel cell components. In yet another commonly owned U.S. Pat. No. 3,964,930 to Carl A. Reiser the coolant carrying tubes are not completely enclosed within the separator plate, but are instead adjacent the electrodes and are exposed to the electrolyte used in the stack. In the stack designs shown in Reiser and in Tuschner the tubes must be made from or at least include a protective coating of material which is stable in the fuel cell electrolyte. Also, as more fully discussed in Tuschner, it is highly desirable that the coolant tubes be completely surrounded by a thermally conductive material to aid in the transfer of heat from the cells to the coolant within the tubes. Tuschner describes a grease-like material comprised of a phosphoric acid base with a graphite filler which is stable in phosphoric acid and which is thermally conductive.
While the ability to transfer heat from the cells to the coolant flowing through the tubes is, of course, very important, it is also extremely important that the tubes do not corrode in the phosphoric acid environment of the cell. In both Tuschner and Reiser protection from corrosion is entirely dependent upon the material from which the coolant tube is made, or, more likely, upon the integrity of a protective coating on the tube itself. Typically the tubes may be made from copper which has been coated with a thin layer of polytetrafluoroethylene or perfluoroalkoxy (PFA). The latter coating (PFA) presents a special problem in that it begins to soften excessively if subjected to temperatures above about 630.degree. F.
It would be highly desirable to prevent the phosphoric acid electrolyte from coming into contact with the coolant tubes. This would eliminate the risk of corrosion. If the heat transfer caulking material surrounding the tube did not absorb phosphoric acid, or if it absorbed very little phosphoric acid, this would at least reduce the risk and/or rate of corrosion. Available caulking materials with suitable thermal conductivity properties absorb unacceptably large amounts of phosphoric acid. Prior to the present invention there has been no known caulking compound with the combination of high thermal conductivity and low rate of phosphoric acid absorption, as well as the ability to retain its sealing characteristics and other essential properties in the fuel cell environment.